Cirrus-logic AN379 Manual do Utilizador descarregar pdf (Página 8)

AN379

8 AN379REV2

The Buck Topology

Figure 3 illustrates a typical implementation of a buck

converter using the CS1680 controller. The load is

composed of a string of LEDs. Diode D

BUCK

is the

catch diode, also known as the freewheeling diode,

and its function is to allow the current to flow in

inductor L

BUCK

and to the load after FET Q

BUCK

turns

off. Capacitance C

represents the combined

parasitic capacitance associated with the FET drain

node, usually consisting of the FET drain-source

capacitance, inductor winding capacitance, and

diode D

BUCK

reverse bias capacitance. For a non-

tapped buck topology the leakage inductance L

not a concern.

The buck stage is supplied by the boost output

voltage. The boost output voltage is regulated within

15% by the boost stage. The buck control loop

regulates the output current as long as the peak

current has sufficient margin to rise 10% at the lowest

boost output voltage. Figure 4a and Figure 4b

illustrate idealized waveforms of the buck power FET

BUCK

drain voltage and current, and catch diode

BUCK

current.

LED+

LED-

OUT

BUCK(Sense)

BUCK

GND

BUCKGD

BUCKSENSE

CS1680

BST

BUCK

OUT

BUCKZCD

BST

BUCK

Figure 3. Buck Converter

Area = V

Diode

ު T2

BUCK

Area = V

OUT

ު T2

BUCK

Area = (V

BST

- V

OUT

)

ު T1

BUCK

BST

Diode

OUT

BST

- V

OUT

Figure 4a. FET Drain Voltage

BUCK

1- '

BUCK

BUCKPK

Figure 4b. Current Through Buck Power FET and Catch Diode

1 2 3 4 5 6 7 8 9 10 11 12 13 ... 30 31

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